The invention relates to brightening additives for use in acid copper electroplating baths and particularly in such baths as used to manufacture printed circuit boards for the electronics industry. More particularly, the invention relates to such brightening additives prepared from dialkylamino-thioxomethyl-thioalkane-sulfonates and the like.
Dialkylamino-thioxomethyl-thioalkanesulfonate compositions have been used for many years in methods for the electrodeposition of metal. During the electrodeposition of metal deposits in the absence of organic brightener and leveler additives, metal deposits often form with a crystalline, matte or burned finish, can vary substantially in thickness from place to place on the deposit, can have pinholes, can have poor elongation properties, and can be low in tensile strength. Such a metal plate can be undesirable from an aesthetic point of view, and can be undesirable in many technological end uses. One major end use of the metal deposit is on a copper plated printed circuit board. The copper deposit on a circuit board should be bright, shiny, lustrous, uniform in thickness, have substantial elongation properties, and modest tensile strength. Bright, shiny boards are easily wetted by solder compositions providing for rapid and efficient connection of electrical components to the printed circuit board. Uniform continuous coatings of copper that have acceptable elongation properties and tensile strength can be resistant to the formation of hairline cracks, which can form during the manufacture and use of the electronic device, that can result in the interruption of the electrical current and in the failure of the electronic device to operate.
A significant industry effort has addressed the need for the preparation of dialkylamino-thioxomethyl-thioalkanesulfonate brightener compositions of high purity at high yield substantially free of inactive or harmful by-products and avoids generating substantial quantities of nonvolatile impurities which can interfere in electroplating. Notable in the industry effort is the method disclosed in U.S. Pat. No. 4,667,049 wherein an alkali metal dialkylamino-thioxomethyl-thioalkane-sulfonate composition is prepared by reacting a monoamine with carbon disulfide to form a dithiocarbamic acid monoamine salt, reacting the salt with an alkyl sultone to form a dialkylamino-thioxomethyl-thioalkane-sulfonate monoamine salt which is then reacted with an alkali metal base to form the alkali metal salt.
In the conventional electroplating process, including such processes for manufacture of printed circuits, a "break-in" period is needed after the brightener is added to the plating bath. During this period the brightening activity builds to a working steady-state level needed to produce bright plated metal. When brightening activity of a working plating bath declines, plating is interrupted to allow for "break-in" of freshly added brightener. Since such "break-in" periods typically range from several hours to several days, the efficiency or through-put of the plating operation is limited. There is a need in the plating industry, to substantially reduce or eliminate brightener "break-in" so as to improve the efficiency of plating operations.